Re: [PATCH v3 5/7] xen: add capability to remap non-RAM pages to different PFNs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.09.2024 12:39, Juergen Gross wrote:
> When running as a Xen PV dom0 it can happen that the kernel is being
> loaded to a guest physical address conflicting with the host memory
> map.
> 
> In order to be able to resolve this conflict, add the capability to
> remap non-RAM areas to different guest PFNs. A function to use this
> remapping information for other purposes than doing the remap will be
> added when needed.
> 
> As the number of conflicts should be rather low (currently only
> machines with max. 1 conflict are known), save the remap data in a
> small statically allocated array.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with two cosmetic remarks:

> @@ -792,6 +793,70 @@ int clear_foreign_p2m_mapping(struct 
> gnttab_unmap_grant_ref *unmap_ops,
>       return ret;
>  }
>  
> +/* Remapped non-RAM areas */
> +#define NR_NONRAM_REMAP 4
> +static struct nonram_remap {
> +     phys_addr_t maddr;
> +     phys_addr_t paddr;
> +     size_t size;
> +} xen_nonram_remap[NR_NONRAM_REMAP] __ro_after_init;
> +static unsigned int nr_nonram_remap __ro_after_init;

I take it this is the canonical placement of section attributes in the
kernel? (In Xen I'd ask for the attributes to be moved ahead of the
identifiers being declared.)

> +/*
> + * Do the real remapping of non-RAM regions as specified in the
> + * xen_nonram_remap[] array.
> + * In case of an error just crash the system.
> + */
> +void __init xen_do_remap_nonram(void)
> +{
> +     unsigned int i;
> +     unsigned int remapped = 0;
> +     const struct nonram_remap *remap = xen_nonram_remap;
> +     unsigned long pfn, mfn, end_pfn;
> +
> +     for (i = 0; i < nr_nonram_remap; i++) {
> +             end_pfn = PFN_UP(remap->paddr + remap->size);
> +             pfn = PFN_DOWN(remap->paddr);
> +             mfn = PFN_DOWN(remap->maddr);
> +             while (pfn < end_pfn) {
> +                     if (!set_phys_to_machine(pfn, mfn)) {
> +                             pr_err("Failed to set p2m mapping for pfn=%lx 
> mfn=%lx\n",
> +                                    pfn, mfn);
> +                             BUG();

Wouldn't panic() get you both in one operation? Or do you expect the call
trace / register state to be of immediate relevance for analysis?

Jan